Tandem drive axle assemblies having a forward rear axle and a rearward rear axle in proximity with each other are well known. Such tandem drive assemblies are widely used on heavy duty trucks and other over-the-road vehicles, such as busses, which have a high vehicle weight and/or a high load carrying capacity. In such assemblies, both rear axles may be power driven.
An inter-axle differential (IAD) is commonly employed in such vehicles to split the input shaft torque between the front and rear axle of the tandem. It is common for a vehicle operator to engage and disengage a lock out that overrides or disables the IAD through the use of a pneumatic switch, which typically is mounted on the vehicle dash. The pneumatic switch, in turn, applies air to an axle mounted actuator, which engages a sliding dog clutch to “lock” the inter-axle differential.
However, there are several shortcomings to the above-described manual methods of engaging/disengaging the IAD. First, failure of the vehicle operator to notice that wheel end slip is occurring and then to engage the IAD, can result in spin out failures. Second, engagement of the IAD, while significant slipping is in process, can result in damage to either or both of the drive axles. Third, leaving the IAD engaged for an extended length of time can result in “drive line wind-up” and a resulting inability to disengage the IAD without reversing the vehicle. Fourth, since the actuation of the engagement and disengagement of the IAD are typically switched on or switched off by the operator, the IAD does not smoothly transition from one mode to the other. As a result of these shortcomings, extended wear can occur and the operator may not notice the wear, as actual engagement and disengagement of the IAD is not typically indicated to the operator.
More recently, automatic differential lockout mechanisms have come into use that attempt to minimize wear from engaging or disengaging of the differentials. U.S. Pat. No. 4,671,373 to Sigl discloses a locking-type differential being automatically prevented from engaging upon certain vehicle operating conditions. For example, if the steering wheel is deflected or transverse acceleration is sensed, the differential will not be allowed to be locked. The differential is likewise unlocked or prevented from locking if, for example, the brakes are applied or the engine is operating under an idle condition. On the other hand, for example, if a kick-down signal from the accelerator is sensed, or if the difference in speed of driven and rolling wheels exceeds a predetermined amount, the differential is controlled to lock.
U.S. Pat. No. 5,071,392 to Stall et al. discloses a process of continuously controlling the degree of locking of open differential drives in a driven axle of a multi-axle vehicle where the differential speed of the driven wheels is compared to the vehicle speed of the non-driven wheels.
U.S. Pat. No. 5,130,928 to Petersen provides an anti-lock and/or anti-slip apparatus for commercial type vehicles where an electronic system determines whether the rotational speed of the cardan shaft varies from the average speed of the monitored wheels. Upon receipt of a variance, the electronic system controls the locking of a longitudinal differential.
U.S. Pat. No. 5,989,147 to Forrest et al. discloses an electronically controllable differential which transfers a predetermined amount of torque through a rotatable differential based upon, in part, a predetermined rotational condition of the side gear.
U.S. Pat. No. 6,174,255 to Porter et al. teaches a differential lock control system that employs speed sensors and an articulation angle sensor that communicate speed signals and an articulation angle signal to a microprocessor for controlling the locks on front and rear differentials for an articulated work vehicle. In an automatic mode, the microprocessor controls the locking of the differentials by comparing predicted axle speeds to actual speeds received from the speed sensors and an articulation angle from the articulation sensor.
U.S. Pat. No. 6,487,486 to Anderson discloses a process for continuously controlling the transfer of torque within a differential while utilizing control logic methodology. Changes in torque values are determined from output shafts speeds and vehicle speeds.
U.S. Pat. No. 6,579,204 to Brown et al. generally discloses a full-time power transfer system for controlling speed differentiation and torque biasing across an interaxle differential in response to changes in sensors that monitor dynamic and operational characteristics of the vehicle.
Even with the above-described current automatic means for controlling the engagement and disengagment of the inter-axle differential, optimization of conditions, for example, minimizing damage and excessive wear of IAD components, improving the engagement/disengagement timing of the inter-axle differential locking mechanism, and enhancing the actuation of the engagement/disengagement, can still be made.